The present invention relates to an ink jet recording device for emitting ink droplets to record an image, and more particularly to an ink jet recording device for emitting ink droplets by individually driving a plurality of ultrasonic generating elements to generate ultrasonic beams.
The ink jet recording device which emits a liquid ink as small droplets onto a recording medium to record an image has many advantages such that the image can be directly recorded on plain paper. In the ink jet recording device, however, the liquid ink is likely to concentrate because of evaporation or volatilization of a solvent for liquid ink. Therefore, there is a disadvantage in that a nozzle for emitting ink droplets is clogged, thereby causing a trouble in emitting ink droplets.
In a so-called line-type head which is provided with a number of nozzles, it is inevitable to prevent the clogging of nozzles. As a consequence, there is such a disadvantage that it is necessary to provide additional means for cleaning the nozzles, or nozzles having small diameters cannot be used. In other words, in the conventional ink jet recording device, it is more difficult to use the line-type head having a lower reliability and a higher resolution.
In order to overcome these disadvantages, there has been proposed a method for emitting ink from a liquid ink surface by using the pressure of the ultrasonic beams which are generated by a piezoelectric element array comprising a thin film piezoelectric material.
The method is a so-called nozzle-less method which does not require a partitioning wall for a nozzle or an ink flow channel for each of individual dots. As a consequence, the method is advantageous in the prevention of the clogging of nozzles which causes a large trouble in forming a line head. The method is suitable for an increase in the resolution because ink droplets having very small diameters can be stably generated and emitted.
However, since this method converges ultrasonic beams in the ink by using an acoustic lens having a diameter larger than a recorded picture point or a recording resolution (for example, the diameter is thirty times as large as the recorded picture point), an interval for emitting ink droplets must be set to a wide level. In order to reduce the interval for emitting ink, a recording head is normally constituted by arranging a plurality of piezoelectric element arrays in a zigzag manner. However, because of the zigzag arrangement thereof, a periodical disuniformity in concentration and a slight deviation in position between adjacent dots affect the image quality.
Therefore, there has been proposed, as another method for converging ultrasonic beams disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-18443, a method which is referred to as the sector electronic scanning method in which the respective piezoelectric elements at piezoelectric element array which is constituted by arranging a plurality of piezoelectric elements at predetermined intervals are driven at predetermined delay time differences thereby exciting ultrasonic waves whose phases are controlled to converge the ultrasonic waves onto one point in the vicinity of the ink liquid surface. According to this method, it is possible to perform recording images in a high resolution by changing emission positions for ink droplets without being restricted by an arrangement pitch of the piezoelectric element array.
However, in the sector electronic scanning method, in order to emit an ink droplet at each of desired positions on the ink liquid surface, it is necessary to control the piezoelectric elements by calculating accurately delay time differences for driving the piezoelectric elements so that the ultrasonic beams are converged on each of the positions with the result that the driving circuit becomes complicated and expensive. In a case where an ink droplet is emitted at a position other than one point on the ink liquid surface which is aligned with the central position of the piezoelectric element array, the converged ultrasonic waves have a directionality with the result that the ink droplet is emitted in a slanting direction to the ink liquid surface. As a result, there is a disadvantage in that the attainment position of the ink droplet is deviated due to a change in a height of the ink liquid surface or a distance between the ink liquid surface and a paper.
With respect to these disadvantages, there has been proposed a method which is referred to as a linear electronic scanning method in which a predetermined number of piezoelectric elements (a group of piezoelectric elements) in the piezoelectric element array are simultaneously driven to emit one ink droplet, and the position of the group of piezoelectric elements which are simultaneously driven is shifted sequentially to shift the emission position for ink droplet. In this method, since the delay time for each piezoelectric element is set to be symmetrical in a right and a left directions with respect to the center of the group of piezoelectric elements, each ink droplet is always emitted straightly in a direction perpendicular to the liquid surface. As a method for solving the troubleness in the control of the delay time, there has been proposed a more simple method in which the piezoelectric array is divided into two groups on the basis of the Fresnel's diffraction theory and the delay time is set so that the phases of the two groups are different from each other by a half wave-length.
In the piezoelectric element array driving circuit for realizing such a linear electronic scanning method, the group of piezoelectric elements which should be driven on the basis of image data must be selected from the piezoelectric element array. In other words, in the linear electronic scanning method, a plurality of piezoelectric elements must be simultaneously driven with respect to the image data corresponding to one imaging point. As a consequence, a driving circuit having a special structure is required which is completely different from the structure of a normally used ink jet recording device in which one driving element is allowed to correspond to one image data.
In the ink jet recording device according to the conventional linear electronic scanning method, ink droplets are emitted at the same interval as the arrangement pitches of the piezoelectric element array. Since it is difficult to emit ink droplets in pitches narrower than the arrangement pitches, an increase in the recording resolution is restricted.
In order to improve a recording speed in the linear electronic scanning method, it is desired that driving data for driving each of the piezoelectric elements by the driving circuit can be appropriately set in an efficient manner.
In a thermal head which is used in a conventional ink jet recording device utilizing a method of emitting an ink droplets by using pressure of a bubble generated, for example, by heat, or in a conventional thermal transfer recording device, there has been known a method for shortening a time for transmitting driving data for recording one line by using a shift register which has the same number of steps as the number of heat generation elements which constitute the thermal head, dividing the shift register into a plurality of groups, and providing input ports for the respective groups to input driving data simultaneously from these input ports.
However, the driving data transmission method in which driving data is input simultaneously from a plurality of input ports cannot be applied as it is to the ink jet recording device according to the linear electronic scanning method. In other words, in the linear electronic scanning method, a plurality of piezoelectric elements must be allocated to one image data with the result that there is generated a case in which driving data corresponding to one image data is input over two adjacent input ports. As a consequence, when an attempt is made to shorten the transmission time of driving data by providing a plurality of input ports, a complicated processing must be carried out for generation of the driving data, or a driving circuit in which the driving data is transmitted in a complicated manner must be constituted.
In a case where data is recorded particularly at a high speed, each of piezoelectric elements is continuously or intermittently driven frequently over a long time. Consequently, ultrasonic waves are not sufficiently generated because of the generation of heat in the piezoelectric elements, and ultrasonic waves having desired frequencies are not generated with the result that the emission of ink droplets becomes unstable, and the piezoelectric elements are deteriorated in a short period.
In a line head in which thousands of piezoelectric elements are arranged, there are some cases where the number of elements which are driven simultaneously increases and the capacity of the power supply for the driving circuit becomes large.
As described above, the ink jet recording device utilizing a method for emitting ink droplets by driving piezoelectric element array according to the linear electronic scanning method is capable of recording without clogging in principle. On the other hand, since it is necessary to drive a plurality of piezoelectric elements simultaneously according to the image data to be recorded and emit one ink droplet, there is a subject in that how the driving circuit is simply constituted.
The ink droplets are emitted at the same intervals as the arrangement pitches of piezoelectric element array, and it is difficult to emit ink droplets in pitches narrower than the arrangement pitches.
Therefore, there is a limit in increase in the resolution for recording.
Furthermore, since it is necessary to emit one ink droplet according to the image data to be recorded by driving a plurality of piezoelectric element at the same time, it is a subject how is a driving circuit constituted.
Particularly, with an elongated line type head, it is desired that driving data is input and set simultaneously from a plurality of input ports in order to improve a recording speed. There occurs a case in which driving data corresponding to one image data is input over two adjacent input ports. Therefore, a complicated processing must be carried out for generation of driving data, and a driving circuit in which driving data is transmitted in a complicated manner must be constituted so that there is a disadvantage in that it is difficult to set appropriate data in a simple structure.
For such a reason that heat is generated by repeatedly driving the piezoelectric elements over a long time, ultrasonic waves are not sufficiently generated and ultrasonic waves having the desired frequency cannot be obtained with the result that it is feared that the emission of ink droplets becomes unstable, the image quality is lowered, and the piezoelectric elements are deteriorated in a short period. There is also a disadvantage in that the capacity of power supply for the driving circuit increases with an increase in the number of piezoelectric elements which are simultaneously driven in the line head.